Mixing bacteria with drinking water sounds like a recipe for an upset stomach. But a bug-powered desalination cell that takes salt out of seawater may cut the cost of quenching the world's thirst.
With one-third of the planet's population lacking sufficient drinking water, governments are increasingly looking to desalination to produce fresh water from seas and estuaries.
However, conventional desalination plants consume large amounts of energy. For instance, they use reverse osmosis, in which water is forced at enormous pressure through membranes that screen out salt. This means there's keen interest in less energy-intensive approaches.
One that's recently been explored uses bacteria that generate electrical power by eating organic matter. If these bacteria feast on domestic sewage in an "anode" chamber, they generate electrons that can pass into a circuit while releasing protons. To balance the now positively charged sewage solution, negative chloride ions squeeze through a membrane from an adjacent chamber containing salty water that is to be desalinated.
Meanwhile the electrons are delivered to a third, "cathode" chamber on the opposite side of the desalination chamber. The cathode chamber is also filled with a saltwater solution. Here, the electrons react with hydrogen ions in the solution and oxygen from the air to form water. To balance the negative charge caused by the loss of positive hydrogen ions, sodium ions pass from the central saltwater chamber into the cathode chamber via another membrane. With time, the salty water in the central chamber becomes fresher.
Tang trouble
But such a device is relatively inefficient, says Bruce Logan at Pennsylvania State University in University Park: as the organic content in the waste water falls, the voltage produced by the bacteria drops and pulls fewer ions out of the saline water, leaving it with a salty tang. Flushing the anode with more sewage is one option, but Logan's team are keen to squeeze as much use from each batch of dirty water as possible.
They have has developed a simple solution: boost the voltage from the bacteria with an external power source to make up any deficit. Furthermore, if the electrons react only with water at the cathode, they generate hydrogen gas – which contains enough energy to fuel the extra voltage requirements.
The team filled the central chamber of their cell with brackish water containing 5 grams of sodium chloride per litre, as might be found in an estuary, and applied a voltage of 0.55 volts to the setup. Over several hours, the salinity of the salt water dropped by 68 per cent to 1.6 grams of salt per litre. A standard microbial desalination cell stalls when the salinity reaches 40 to 60 per cent – or 2 to 3 grams of salt per litre.
By varying the voltage added to the system as the reaction continues – and by using more water in the anode and cathode chambers than in the saltwater chamber – Logan says it should be possible to reduce the salinity to the 0.8 grams of salt per litre typical of drinking water.
"It is likely people would still want some sort of added treatment to ensure good quality water, and thus we expect a downstream reverse osmosis unit to still be used," he says, but using the new cell as a "pre-treatment to greatly reduce salt concentrations" should help to substantially reduce the energy needed to obtain fresh water from the sea.
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